This invention relates to a manufacturing method of compound semiconductor crystals and to an apparatus for carrying out this method.
Recently, because of their large mobility at room temperature compared with Si single crystals, GaAs single crystals have come into widespread use as the substrate for super high speed ICs. For use as the IC substrate, there is desired a crystal that is semi-insulative and exhibits uniformity of electric properties. Especially, the undoped semi-insulative GaAs single crystal which has higher purity and shows high mobility and which is made by the direct compound LEC (liquid-encapsulated Czochralski) method using a PBN (pyrolitic boron nitride) crucible is undergoing a vigorous development.
Concerning this undoped crystal, however, the quality of being semi-insulative is sensitive to the composition. For instance, the relationships between the properties of the crystal and the composition ratio are well known (R. N. Thomas et al, "Semiconductors and Semimetals" Vol. 20, chapter 1.1, Academic Press, New York (1984)).
FIG. 1 shows the relationship between the melt composition and resistivity as well as mobility of GaAs. From this figure, it is understood that the resistivity and mobility are low in the Ga-rich compositions. In another example, the relationship between the solidified ratio of the GaAs melt and the resistivity is shown in FIG. 2 for several melt compositions. FIG. 2 shows that a crystal made from a melt in which the Ga/As mole ratio is greater than 1 does not become semi-insulative for the whole ingot. Furthermore, as shown in the following, a crystal made from a Ga-rich composition is poor in thermal stability, so that its use for semiconductor devices such as MESFET or integrated devices is not preferable. Concerning this point, FIG. 3 shows the relationship between the GaAs melt composition and the resistivity for various annealing conditions after growth. The thermal stability decreases as the Ga/As melt composition ratio exceeds 1.
As described in the previous examples, the melt composition is extremely important for the growth. Therefore, pulling of a GaAs single crystal is usually performed by setting the initial melt composition at a slightly As-rich (Ga/As mole ratio.ltoreq.1) condition.
However, uniformity of electrical properties along the pull up axis is not sufficient, and the unevenness of the electric properties in the resulting wafer plane increases as the solidifying proceeds. Furthermore, polycrystallization is observed to a large degree. These effects occur because the As ratio in the As-rich GaAs melt increases as the solidifying proceeds.
On the other hand, an attempt has been made by introducing As vapor into a Ga-rich melt to control the GaAs melt composition (32th Spring Conference, p. 724, 1985, Japan Applied Physics Society). In this attempt, the tip opening of an ampoule containing As lumps is inserted upside down into the surface region of a GaAs melt, and then the ampoule is heated with a heater during the pulling process. This experimental result showed that the crystal is grown in the form of a Ga-rich to As-rich crystal along the GaAs ingot. Furthermore, it is difficult to detect the GaAs melt composition and the amount of introduced As during the pulling step because of the use of the As vapor, and it is thereby also difficult to control the composition of GaAs melt.